The present invention relates to a method for pulling up a single crystal of semiconductor, such as silicon or the like, and more specifically to a method for pulling up a single crystal having a heavy weight, in which case, the single crystal can be grown in the dislocation-free state without any accidents of the single crystal dropping, thereby ensuring to stably produce the single crystal in a high crystalline quality.
In recent years, silicon single crystals, which are used to form a circuit device, such as a large-scale integrated circuit (LSI""s), have been mostly produced by means of the Czochralski method (hereafter referred to as xe2x80x9cthe CZ methodxe2x80x9d). Such a single crystal grown by the CZ method is usually pulled up from a molten material in a quartz crucible, and oxygen is included in the crystal. This provides excellent properties regarding the substrate quality in the process of manufacturing LSI""s.
In producing a single crystal using the CZ method, the pulling rate of the crystal has close relation to the temperature gradient in the single crystal. A greater temperature gradient provides acceleration in the pulling rate. Moreover, an inert gas is always supplied at the area around the crystal in the course of pulling up the crystal, and it is necessary to regulate the gas flow in order to avoid the turbulence, stagnation, convention and the like in a chamber for the crystal growth. In the case of industrially producing silicon single crystals, a flow-regulating member is normally disposed around the single crystal to be pulled, thereby allowing the heat of radiation to be a shield and the inert gas flow to be regulated.
FIG. 1 is a longitudinal sectional view of an apparatus for pulling up a silicon single crystal equipped with a flow-regulating member by the CZ method. A crucible 2, which is used to produce the silicon single crystal, is normally disposed in the center of the apparatus 1 for pulling the single crystal. The crucible 2 has a dual structure such that it comprises an inner quartz crucible 2a and an outer graphite crucible 2b. A heater 3 made of graphite is disposed outside the crucible 2 to fuse silicon into molten silicon 4 inside the crucible 2. A pulling wire 5 is used to pull up the single crystal, and a seed crystal 6 is attached onto one end of the pulling wire 5. The seed crystal 6, the lower end of which comes in contact with the surface of the molten silicon material 4, is pulled upwards and thus the single crystal 7 is grown by solidifying the molten material onto the lower end portion of the seed crystal.
In this case, a flow-regulating member 8 is disposed above the molten material 4 in such a way that it surrounds the silicon single crystal to be grown. The flow-regulating member 8 intercepts the heat of radiation emanating from both the heater 3 and molten material 4, thereby increasing the temperature gradient in the pulled single crystal 7. In the process of crystal growth, a high purity argon gas is always supplied into the chamber to form gas flow 31 (indicated by arrows in the drawing). The gas flow 31 thus formed is regulated by the flow-regulating member 8. The distance between the lower end of the flow-regulating member 8 and the surface of the molten material 4 stored in the crucible (hereafter the distance being simply referred to as the xe2x80x9cgapxe2x80x9d) is normally determined to be 15-30 mm, taking into account the effects of both shielding the heat of radiation and regulating the inert gas flow.
In pulling up the single crystal by the CZ method, it is necessary to remove residual dislocations in the seed crystal as well as dislocations resulting from the thermal stress, when the seed crystal comes into contact with the molten material, and thus to prevent the effect of these dislocations from extending into the main body portion (body) of the single crystal. In order to produce a dislocation-free single crystal by eliminating dislocations from the crystal surface, the so-called xe2x80x9cDash neck processxe2x80x9d is employed, in which case, the end of the crystal is tapered.
In the general Dash neck process, the crystal grown from the seed crystal is tapered into a very small diameter of 3 to 4 mm, so that the residual dislocations in the seed crystal and dislocations generated by the contact with the molten material can be moved outside the crystal, thereby enabling a dislocation-free single crystal to be grown. For this purpose, the neck portion is formed just after starting to pull up the crystal. When, however, the diameter at the neck portion exceeds 5 mm, the dislocations can hardly be moved to the outside the crystal, thereby making it difficult to produce the dislocation-free single crystal.
As described above, a diameter of not more than 4 mm at the neck portion enables the dislocations to be eliminated in a high efficiency. However, if the diameter at the neck portion is set to be too small, a high strength for supporting the single crystal ingot cannot be obtained. As a result, a fracture takes place at the neck portion in the process of pulling up the crystal or in the subsequent processes of cooling and removing the ingot, and further there is a possibility that the single crystal drops into the molten material in the crucible, thereby causing the apparatus for pulling up the single crystal to be damaged, the overflow of the molten material, steam explosion and other effects to take place, and causing a possible accident resulting in injury.
In the Dash neck process, a desired neck shape can be formed by controlling the pulling rate and the temperature of the molten material. An increase in the pulling rate or an increase in the temperature of the molten material provides a smaller diameter at the neck portion, whereas, a decrease in the pulling rate or a decrease in the temperature of the molten material provides a greater diameter at the neck portion.
However, even if the diameter at the neck portion is controlled into a fixed value, the crystal shape at the neck is altered when an unexpected disturbance occurs in the temperature of the molten material in the crucible. For instance, a sudden increase in the temperature of the molten material in the process of pulling up the single crystal provides a much smaller diameter at the neck portion, compared with the preset target value, or at worst it causes the single crystal to be separated from the surface of the molten material. In such a case, the neck portion is again immersed into the molten material and the Dash neck process has to be repeated.
The weight of the single crystal to be pulled up is conventionally limited to 100 Kg or so. In recent years, a high efficiency in the manufacture of semiconductor devices has been strongly required to increase both the diameter and the total length of single crystals, so that the weight of the single crystal tends to exceed 200 Kg. If the diameter of the single crystal at the neck portion is increased so as to avoid the above-mentioned problem, it is difficult to eliminate dislocations from the single crystal in the Dash neck method, as described above, so that the work to re-form the neck portion is frequently repeated. Along with an increase in the weight of the single crystal, when it is preferentially intended to prevent the neck portion from being damaged or to prevent the single crystal from dropping, it is difficult to eliminate dislocations therefrom, thereby increasing the number of the re-works in the Dash neck process and thus reducing the efficiency conspicuously in the process of the single crystal.
To overcome the above problem resulting from the increase in the weight of the single crystal, the present applicant proposed a method for producing a single crystal with an increased gap in Japanese Patent Application Laid-open No. 11-189488 (hereafter this method is simply abbreviated as the xe2x80x9cgap-increased methodxe2x80x9d). The apparatus for pulling up a single crystal was designed such that the distance between the lower end of a flow-regulating member surrounding the single crystal to be pulled up and the surface of a molten material in a crucible was greater than that in the conventional method using the CZ method, and the single crystal was pulled up in the state in which a seed crystal is in contact with the molten material, after the single crystal is heated to provide a small difference in temperature between the lower end of the seed crystal and the molten material.
In the xe2x80x9cgap-increased methodxe2x80x9d thus proposed, the heat of radiation from the heater to the seed crystal increases, and therefore the temperature difference between the lower end of the seed crystal and the surface of the molten material can be decreased. As a result, the thermal stress at the moment of contact is reduced, thereby enabling the number of dislocations to be reduced at this moment.
Moreover, an increase in the gap provides an increase in the amount of radiation emanating from the heater to the neck portion during pulling. This provides various effects: the temperature gradient at the neck portion decreases; the isothermal lines become flatter; the temperature gradient at the neck portion in the radial direction decreases; the thermal stress decreases and at the same time the traveling speed of the dislocations decreases, hence enabling the dislocations at the neck portion to be removed at a high efficiency. Accordingly, these effects allow the dislocations to be removed even in the case of an increased diameter at the neck portion, thereby making it possible to pull up the single crystal having an increased weight without any propagation of dislocations.
In accordance with the gap-increased method proposed therein, furthermore, the conditions of thermal shield for the heat of radiation emanating from the molten material and heater in the course of growing the single crystal as well as the thermal history can be varied.
It is recognized that the thermal history during the crystal growth provides a strong effect on the formation of micro-defects in the single crystal and the properties of the micro-defects to be formed can be varied by the thermal history. For instance, when the single crystal is pulled up with an increased gap, the micro-defects can hardly be formed, although the pulling rate cannot be increased, whereas when the single crystal is pulled up with a decreased gap, the pulling rate can be accelerated and it is possible to form single crystals including micro-defects of different type. Regarding the quality of finished crystals, furthermore, there are various requirements of users on micro-defects in the crystal and therefore a variety of conditions for growing a single crystal is employed to meet these requirements.
Taking the above matters into account, the present invention is achieved. The objects of the present invention are to provide a method for pulling up a single crystal, which allows to grow a dislocation-free single crystal under the prerequisite conditions of pulling up the single crystal having a heavy weight, even in the case of a flow-regulating member being disposed with a greater gap than those in the normal one, to provide a method for pulling up a single crystal, which enables a single crystal to be securely pulled up without any accident of dropping, and to provide a method for pulling up a single crystal, which provides a high degree of freedom in selecting the conditions of crystal growth.
To attain the above objects, the present inventors investigated to determine a suitable diameter of the single crystal at the neck portion and to determine the conditions for crystal growth. Regarding the selection of the diameter at the neck portion, the present inventors have found that the accident of a single crystal dropping can be avoided, and a dislocation-free single crystal can be obtained, when the diameter at the neck portion is set to be not less than 6 mm.
Actually, when a single crystal having a diameter of 3 to 4 mm as a target diameter is produced with the conventional Dash neck process, a sudden change in the temperature of the molten material makes it difficult to control the diameter at the neck portion, and a single crystal having a smaller diameter compared with a preset target value is usually formed. In fact, it is often encountered to obtain a single crystal having a diameter of less than 3 mm. In this case, assumed that a maximum possible tensile strength at the neck portion is 20 Kg/mm2, the weight at which the single crystal can be pulled up is less than 141 Kg or so, hence making it impossible to support a single crystal having a weight of more than 200 Kg. In this case, therefore, it is necessary to again re-form the neck portion after melting the crystal.
On the contrary, when the diameter at the neck portion is set to be not less than 6 mm as a target value, the minimum diameter of 5 mm can be obtained even if the crystal having a smaller diameter than the target value by 1 mm is formed in the same circumstance of processing. In this case, the weight at which the single crystal can be pulled up is not less than 392 Kg and there is no problem in supporting a single crystal having a weight of more than 200 Kg, hence enabling the process of pulling up to securely continue without any trouble of dropping.
However, if the shoulder portion and body portion are produced using the same increased gap after the formation of the neck portion in the gap-increased method, this leads to a varied thermal history for the single crystal, because the condition of radiation is altered, compared with the conventional CZ method.
A study of the influence of the thermal history on the crystal quality reveals that the crystal can be grown in a stage in which varied conditions of crystal growth can be selectively satisfied by changing the gap in the stage of growth at the shoulder and body portions. In the stage of the growth at the neck portion, an increased gap (first distance D1) is selected to form dislocation-free single crystal without the accident of dropping, whereas in the stage of the growth at the shoulder and body portions, a small gap (second distance D2) is selected to form a single crystal under corresponding condition of growth. With this method, it is possible to securely pull up a dislocation-free single crystal having a heavy weight under varied conditions by correspondingly changing the gap in the stage of growth at the neck portion as well as at the shoulder and body portions.
The gists in the present invention, based on the above findings, are two methods (1) and (2) for pulling up a silicon single crystal as follows:
(1) A method for pulling a silicon single crystal, using an apparatus comprising a crucible for storing a molten material for a single crystal to be grown; a heater for heating the molten material; means for pulling up the single crystal to grow after bringing a seed crystal in contact with the surface of the molten material in the crucible; and a flow-regulating member surrounding the single crystal at the growth area for shielding the heat of radiation and for regulating inert gas flow, comprising the following steps of: setting the distance between the lower end of the flow-regulating member and the surface of the molten material to be a first distance D1 when the seed crystal comes into contact with the surface of the molten material in the crucible; forming the single crystal at the neck portion after the seed crystal is in contact with the molten material; thereafter setting the distance between the lower end of the flow-regulating member and the surface of the molten material to be a second distance D2 where D1 (mm) greater than D2 (mm); and forming the single crystal at the shoulder portion and subsequently forming the crystal at the body portion.
(2) In the above method (1), it is preferable that the speed of moving the flow-regulating member is 0.1 mm/min to 1.2 mm/min, when the distance between the lower end of the flow-regulating member and the surface of the molten material is changed from the first distance D1 to the second distance D2.